U.S. Patent Re. 31,847, reissued Mar. 12, 1985, inventor Luckey, discloses a storage phosphor system in which a storage phosphor is exposed to an x-ray image of an object, such as the body part of a patient, to record a latent x-ray image in the storage phosphor. The latent x-ray image is read out by stimulating the storage phosphor with relatively long wavelength stimulating radiation such as red or infrared light produced by a gas or diode laser. Upon stimulation, the storage phosphor releases emitted radiation of an intermediate wavelength, such as blue light, in proportion to the quantity of x-rays that were received. To produce a signal useful in electronic image processing, the storage phosphor is scanned in a raster pattern by a laser beam deflected by an oscillating or rotating scanning mirror or by a rotating hologon. The emitted radiation from the storage phosphor is reflected by a mirror light collector and detected by one or more light detectors to produce an x-ray image signal. Typically, the storage phosphor is translated in a page or slow scan direction past the laser beam which is repeatedly deflected in a line or fast scan direction perpendicular to the page or slow scan motion of the storage phosphor to form an image signal having a matrix of pixels. The x-ray image signal can then be viewed as a visual image produced on a video monitor or other display device or as a permanent image printed out on film or paper.
In storage phosphor readers it is desirable to provide high emitted light collection efficiency in order to speed up the reading of a storage phosphor and to attain maximum reader performance. There has been proposed a storage phosphor reader having a highly efficient emitted light collector system including an array of five photodetectors (photomultiplier tubes) which receive light either directly emitted from a storage phosphor or reflected by a highly efficient specularly reflective mirror light collector. In such a storage phosphor reader, there is a need to verify that the reader is calibrated to the exposure value of the x-ray source so that the image signal read from the storage phosphor is accurate.